river: fix capsule type loss when converting capsules to any

This makes two changes to fix the loss of the capsule type when a
capsule value is converted to interface{}:

1. Structs with no River tags are no longer considered objects, and are
   now considered capsules.

2. Capsules can be converted if the Go types can be converted (such as
   assigning to an interface).

Fixes grafana#3171.
Related to #2547.

pkg/river/internal/value/decode.go

@@ -339,6 +339,17 @@ func tryCapsuleConvert(from Value, into reflect.Value, intoType Type) (ok bool,
 		}
 	}
 
+	// Last attempt: see if the Go types can be converted.
+	if from.Reflect().CanAddr() && from.Reflect().Addr().CanConvert(into.Type()) {
+		val := from.Reflect().Addr().Convert(into.Type())
+		into.Set(val)
+		return true, nil
+	} else if from.Reflect().CanConvert(into.Type()) {
+		val := from.Reflect().Convert(into.Type())
+		into.Set(val)
+		return true, nil
+	}
+
 	return false, nil
 }
 

pkg/river/internal/value/decode_test.go

@@ -229,7 +229,7 @@ func TestDecode_CustomTypes(t *testing.T) {
 }
 
 type customUnmarshaler struct {
-	Called bool
+	Called bool `river:"called,attr,optional"`
 }
 
 func (cu *customUnmarshaler) UnmarshalRiver(f func(interface{}) error) error {

pkg/river/internal/value/type.go

@@ -126,6 +126,9 @@ func RiverType(t reflect.Type) Type {
 		return TypeObject
 
 	case reflect.Struct:
+		if getCachedTags(t).Len() == 0 {
+			return TypeCapsule
+		}
 		return TypeObject
 
 	case reflect.Func:

pkg/river/internal/value/type_test.go

@@ -38,7 +38,8 @@ var typeTests = []struct {
 	{[...]int{0, 1, 2}, value.TypeArray},
 	{[]int{0, 1, 2}, value.TypeArray},
 
-	{struct{}{}, value.TypeObject},
+	// Struct with no River tags is a capsule.
+	{struct{}{}, value.TypeCapsule},
 
 	// A slice of labeled blocks should be an object.
 	{[]struct {